Final answer:
The boldface type words and definitions in the margins of the text enhance comprehension.
Step-by-step explanation:
The boldface type words and associated definitions in the margins of the text "magic and the brain" impact the reading of the text by enhancing comprehension. When readers come across unfamiliar or important words in the text, the definitions in the margins help clarify their meanings and improve understanding. This allows readers to engage more deeply with the text and grasp the concepts being discussed.